The present invention relates generally to cooling devices, in particular, heat sinks for integrated circuit chips such as central processing units (CPU).
Cooling of electronic devices has become one of the major limiting factors in the performance of such devices. In particular, the central processing unit in a typical personal computer can generate up to 40 watts of heat in an area of less than 4 square inches. The performance of the chip is dictated to a large extent by its speed and the higher the speed, the more heat that is created. The more heat that is created, the slower Ad the chip runs and the shorter the life span of the chip.
Noise is also a major factor in the electronics area. Especially as electronic systems with CPU""s are used more frequently inside the home, the desirability of a quiet system has become more prevalent. A typical method for cooling integrated circuit chips involves one or more air moving devices, e.g. fans, used to circulate cool air through the chip area or an attached heat sink. The more heat that is generated, the more air that has to be circulated. As larger and more rapidly moving fans are installed, the more noise that is generated.
Another aspect of computer systems involves the amount of space taken up by the electronic devices within the system. The tradeoff faced is that as a heat sink has a larger surface area, the more efficiently it will cool an attached device. It is highly desirable to provide a heat sink with a high surface area that can take up less space within a computer system and require a less powerful fan for circulating air therethrough to achieve the desired cooling.
Weight is also an issue with computer components. An extruded heat sink is a solid mass of aluminum and in many personal computer systems, in particular, those with an Intel Pentium Pro(trademark) chip, the heat sink exceeds a quarter pound. As these processing chips change location from board mounts to cartridges, the weight becomes even more of a concern. For example, Intel limits the acceptable weight of a heat sink for its latest chip called Klamath to xc2xd pound (250 grams). This extra weight creates a need for extra mounting force using screws and in many cases extra support beams for the heat sink itself. Without the extra support and mounting force, these heavy heat sinks may dislodge themselves during shipping.
In the manufacture of personal computers, typically multiple fans are used which create noise. Also a heat sink is required for individual chips. The vast majority of these heat sinks are made from extruded aluminum. The aluminum is extruded into a design where thick fins run the length of the extrusion. The extrusion is designed to have a flat plane on the bottom which contacts the semiconductor device. To further increase the surface area of these fins, manufacturers typically xe2x80x9ccrosscutxe2x80x9d through the thick fins in a costly machining process which creates separate pins along the extrusion. Each of these pins has at least four sides of surface area, twice as much as the straight thick fin previous to the crosscut. Extruded aluminum heat sinks face severe limitations. The height of the fin can only be a few times the gap of the distance between the fins because of the extrusion process, and the thickness of the fins is limited by the height because in typical extrusion processes the thickness at the base of the extrusion must be at least ⅕ of the height. Therefore, the higher the fin density for a given area, the shorter the fin height. The highest total surface area achievable for an extruded heat sink with crosscuts in a volume less than xc2xdxe2x80x3 high and 4 sq. inches in area usually tops out at about 25 sq. inches. The amount of fins per inch is limited by the thickness and the height. This limitation on density of fins on an extrusion reduces the efficiency of the heat removal.
To make up for this limitation in density that extrusions have, an extrusion being used for a heat sink goes through many secondary operations after it is extruded. First, in many instances the base of the heat sink is machined for flatness in order to make smooth contact with the heat producing substrate it is attached to. Then the bars of extrusion which are typically 8 feet in length are cut down to a particular size, usually 2 inches for a typical semiconductor. The next step is the crosscutting step. After this, the heat sink is usually stamped in order to form mounting holes and locations and then it is anodized and deburred. In all, making an extrusion is an expensive and lengthy process.
U.S. Pat. No. 5,329,426 (Villani) is an example of a method that may be used to attach such an extrusion directly to a heat generating chip carrier. A spring is used to clip the solid heat sink to a pair of braces supporting the chip carrier package.
As the integrated circuit chips have become faster and hotter, the simple extruded heat sink was not sufficient. Manufacturers have added fans or other air-moving devices to these extruded heat sinks. The fan which generally includes its own housing, is screwed onto the top of the extrusion and forces air through the fins. The fan typically adds about an extra half inch to the total height of the extrusion. Due to the constraints within the typical computer system limiting the total height of the heat dissipation elements along the CPU to about one inch, the extruded heat sink portion is thus limited to a total of xc2xdxe2x80x3 in fin height. To increase the cooling of the CPU, the fan speed has had to be increased to blow more air through the extrusion. The more air passing over a surface, the higher the thermal conductivity because air is pulling away the heat more quickly. Higher air flow is subject to diminishing returns, however, in that doubling the air velocity fails to double the thermal conductivity of the surface. Fans of the 40 mm. style, typically used for CPU heat sinks in a personal computer, spin at speeds as high as 10,000 RPM""s. This high speed creates well over 30 dB""s of noise. Reducing the fan speed will reduce the noise level, but also reduces the performance of the heat sink.
To overcome some of the limitations of extruded aluminum, some manufacturers have turned their attention to folded fin aluminum. A Taiwanese product made by TUV S.A. and called the CPU Cooler has been seen for sale in the United States. The CPU Cooler uses a strip of folded fin aluminum bonded onto a square thermally conductive base adjacent one edge thereof. On top of the base alongside the folded fin aluminum is a fan for directing air horizontally across the base and through the folded fins. The product recommends the use of thermal tape or thermal grease to adhere the thermally conductive base onto the top of a CPU.
U.S. Pat. No. 5,494,098 (Morosas) describes a heat sink with a fan mounted over folded fin aluminum. The process for making this heat sink involves a number of costly steps including brazing the folded fin to fix it to a solid block of aluminum and milling the top portions of the fixed fins to provide openings into the channels formed by the fins.
Embodiments of the present invention are directed to using clamping methods to secure folded fin to a thermally conductive plate to make a heat sink assembly and related fan assemblies. A sheet of aluminum or other heat conductive material is folded into a wave-like pattern forming an alternating series of ridges and troughs (grooves). The folded fin may be used for placement directly on a substrate to be cooled or on a separate conductive base plate to form a heat sink assembly. Where openings in the ridges of the folded fin heat sink are desired, a method for making such a folded fin heat sink involves providing a thermally conductive sheet with holes spaced periodically, folding the thermally conductive sheet to form the ridges and grooves such that the holes appear in the tops of the ridges and clamping the folded thermally conductive sheet to a thermally conductive surface.
The heat sink of an embodiment of the invention includes a thermally conductive sheet folded into at least one set of alternating ridges or grooves. A clamp mechanism is included for forcing the folded conductive sheet against a thermally conductive plate. Increased thermal efficiency can be provided by including openings through a plurality of the ridges on top of the thermally conductive folded sheet. The clamp mechanism includes an abutment portion for pushing the folded thermally conductive sheet against the plate. The abutment portion may be positioned to press against the ridges or the troughs of the folded conductive sheet. The clamp may include a tab that latches beneath the plate to hold the folded sheet clamped against the plate. The clamped heat sink assembly is further provided with an ability to be clamped to a substrate to be cooled. In one embodiment, spring clips are provided for forcing the thermally conductive plate of the heat sink assembly against the substrate to be cooled. Other mechanical clamping mechanisms may be used for attaching the heat sink assembly to the substrate. For example, the tab that clamps the folded fin to the thermally conductive plate can double as the clamp to the substrate. The tab may be provided with an additional bent portion that can be devoted to making the attachment to a substrate to be cooled.
In accordance with an embodiment of a heat sink-assembly of the invention, a cap may be used to clamp the thermally conductive sheet onto the plate. The cap may include an appendage projecting down from the housing and a flange (tab) on the appendage for latching beneath the edge of the plate. An abutment member may be included in the cap for pressing against the tops of a plurality of ridges in the thermally conductive sheet. Furthermore, a compressible gasket may be included between the abutment member and the top of the thermally conductive sheet to insure that clamping pressure is applied to substantially all of the ridges on the sheet. The clamp advantageously enhances the thermal conductivity between the thermally conductive sheet and the plate beneath the sheet.
In accordance with a still further embodiment of a heat sink assembly of the invention, the clamping device is a spring mechanism which applies pressure to the troughs of the folded fin. A plurality of tabs depending from the spring mechanism may be forced into latching engagement beneath an edge of the base plate to tension the spring to press the folded fin against the plate. When the base plate forms a separate heat sink assembly, the assembly is then attached to a substrate to be cooled with the base plate against the substrate. The spring for use in this embodiment of the invention may include a plurality of parallel linkages. Each linkage fits into one of the troughs of the folded fin. In the embodiment, a pair of crossbars are connected, perpendicular to the linkages, to opposite ends of the linkage. Applying pressure to the crossbars moves the tabs until they hook beneath the base plate. Placing the assembled heat sink onto a substrate to be cooled, the tabs can be pushed further until they engage the substrate. According to an embodiment of the heat sink, the base plate has notches which in combination with the tabs of the spring serve to lock the folded fin onto the base plate in three dimensions.
A method for making a heat sink assembly of the invention involves folding a thermally conductive sheet to form alternating ridges on top and troughs on the bottom. A clamp mechanism is placed on the folded thermally conductive sheet. The folded thermally conductive sheet is clamped to a thermally conductive plate forcing the bottom of the folded thermally conductive sheet against the thermally conductive plate to form a clamped heat sink assembly. The heat sink assembly is attached to a substrate to be cooled with the thermally conductive plate in contact with the substrate.
Heat dissipation can be further enhanced by moving air through a heat sink. A heat sink assembly may be arranged so that the fan may be mounted on top of the heat sink while maintaining the entire heat sink apparatus within the footprint of the integrated circuit chip to be cooled. Such an air moving device may be mounted within a cap of the heat sink assembly. In applications where space limitations are more liberal, a cooling fan assembly of the present invention may be used. The cooling fan assembly includes an air moving device, such as a fan, mounted in an opening of a housing. The housing mounts to a heat sink so that the air moving device directs air at the heat sink. The housing includes a baffle which obstructs a portion of the direct air path from the air moving device. The baffle extends over all of the air channels formed by the folded fin. Air moves around the baffle into the channels and then behind the baffle and out of the air channels. The housing may also include a hood extending over the heat sink where air is exiting the air channels. The hood is positioned to discourage the exiting air from recirculating through the air moving device.
A further aspect of the present invention includes the method for making a folded fin heat sink. A thermally conductive sheet is provided with holes periodically spaced. The sheet is folded to form alternating ridges on top and grooves on the bottom where the holes create openings through the tops of a plurality of the ridges. The holes may be advantageously sized and shaped to form a depressed region beneath the air moving device in any shape such as a rectangle, a bowl or a V-shaped groove. The depressed region beneath the fan has been found to improve the effectiveness of the circulation of air through the heat sink. Indexing is performed prior to folding the sheet to appropriately align the holes in the sheet with the ridge portions being formed.
Other objects and advantages of the invention will become apparent during the following description of the presently preferred embodiments of the invention taken in conjunction with the drawings.